Problem: The base of a solid $S$ is the region bounded by the circle $x^2+y^2=16$. $y$ $x$ ${x^2+y^2=16}$ Cross-sections perpendicular to the $x$ -axis are semi-circles. Determine the exact volume of solid $S$.
Explanation: Let's graph the base of the solid. The thin orange rectangle depicts a representative cross-section sitting on the base. The length of the green segment is $2y$. $y$ $x$ $2y$ $(x,y)$ $(x,-y)$ ${x^2+y^2=16}$ Since each cross-section is perpendicular to the $x$ -axis, the independent variable is $x$. If $A$ denotes the area of each cross-section as a function of $x$, the volume $V$ of solid $S$ is $ V=\int_a^b A(x) \,dx$. To determine the area $A$ as a function of $x$, first express $A$ in terms of $y$. Since the semi-circular cross-section rests on the rectangle pictured above, the diameter of the semi-circle is $2y$. The radius of the semi-circle is $y$. $y$ $2y$ The area $A$ of the semi-circle is $A=\dfrac12\cdot\pi y^2=\dfrac\pi2 y^2$. What is $A$ as a function of $x$ ? The corner point $(x,y)$ of the rectangle lies on the circle $x^2+y^2=16$. Let's rewrite the equation as $y^2=16-x^2$. Now we can express $A=\dfrac\pi2 y^2$ in terms of $x$ as $A(x)=\dfrac\pi2\left(16-x^2\right)$. Can you express the volume $V$ of solid $S$ as a definite integral? Since $x$ goes from $-4$ to $4$, the volume formula $ V=\int_a^b A(x) \,dx$ gives us the definite integral $ V=\int_{-4}^4 \dfrac\pi2\left(16-x^2\right) dx=\dfrac\pi2\int_{-4}^4 \left(16-x^2\right) dx$. Since we're integrating an even function over a symmetric interval, we can rewrite the integral as $ V=\pi\int_0^4 \left(16-x^2\right) dx$. What is the value of the integral? $\begin{aligned} V&=\pi\int_0^4 \left(16-x^2\right) dx \\\\ &=\pi\left[16x-\dfrac13x^3\right]_0^4 \\\\ &=\pi\left[16(4)-\dfrac13(4)^3-\left(16(0)-\dfrac13(0)^3\right)\right] \\\\ &=\pi\left[64-\dfrac{64}3\right] \\\\ &=\dfrac{128}3\pi \end{aligned}$